1. Field of the Invention
The present invention relates to an optical filter (optical wave multiplexer/divider) for use in wavelength division multiplexed transmissions.
2. Description of the Prior Art
&lt;Prior Art Embodiment 1&gt;
First, a first prior art embodiment will be described with reference to FIG. 1.
FIG. 1 is a schematic view of a prior art optical filter (optical wave multiplexer/divider) for wavelength division multiplexed communications.
In FIG. 1, the reference numerals 01 and 02 indicate slab waveguide lenses, 03 is arrayed waveguides, 04 is an input waveguide or input waveguides, and 05 is an output waveguide or output waveguides.
In this case, to appropriately operate the arrayed waveguide grating type optical filter, it is required that in the arrayed waveguides connecting the slab waveguide lenses 01 and 02, the connection point with the lens is on the extension of the straight line radially drawn from the input/output focus of the slab waveguide lens, and lengths of all waveguides adjacent to each other differ by a constant value required in design, and the lengths geometrically increase or decrease monotonically.
The above arrayed waveguides 03, as shown in FIG. 1, is arrayed waveguides 03 each of which comprises a straight waveguide 03a--a bend waveguide 03b--a straight waveguide 03c, which are individually connected.
It is known that characteristics such as optical loss, crosstalk, and the like in the arrayed waveguide grating are improved with decreasing length of the arrayed waveguides.
In view of the fact, it is desirable that a shortest waveguide length is zero, however, the practical bend waveguide radius must be greater than a constant value determined by the waveguide structure, and the shortest waveguide length is a finite value from the requirement of optical waveguide layout.
The arrayed waveguide grating type filter shown in FIG. 1 is experimentally known that the shortest waveguide length can be practically reduced and, as a result, have superior characteristics.
However, the above arrayed waveguide grating type filter is defective in that, in the lower waveguide and the upper waveguide of the arrayed waveguides 03, the upper one is apparently longer in length.
On the other hand, in a filter of wider wavelength spacing, it is required to set the optical length difference to a very small value.
Specifically, where d.sub.0 is a difference in length between adjacent waveguides of the above arrayed waveguides 03, .lambda..sub.0 is a central wavelength, and n.sub.g is a group refractive index, a free spectral range (FSR) is represented by the formula (1). EQU Wavelength spacing.times.channel number.apprxeq..lambda..sub.0.sup.2 /(d.sub.0 .times.n.sub.g)=FSR (1).
Therefore, when the required wavelength spacing is wide or a plurality of channels are required, it is necessary to be set d.sub.0 to a very short value.
However, as described above, due to the geometrical limitation, since in the prior art embodiment there is a lower limit of optical length difference that can be disposed, there is a case where setting the optical length difference to a very short value cannot be geometrically found or, even if achieved, the size of the circuit becomes extraordinarily large. If such a device is to be achieved by a waveguide type, since the size of usable substrate material has a certain limit, it is practically impossible to produce such a device when the circuit size exceeds the limit.
Therefore, it has been difficult to achieve with this construction an arrayed waveguide grating with a wide FSR (Free Spectral Range) which is required to be set to a very short optical length difference, that is, an arrayed waveguide grating with wide in wavelength spacing for separation and multiplexing or a plurality of channels.
&lt;Prior Art Embodiment 2&gt;
Next, a second prior art embodiment will be described with reference to FIG. 2. Japanese Patent Application Laying-open No. 6-27339 (corresponding U.S. Pat. No. 5,212,758) discloses an S-formed optical waveguide as the second prior art embodiment.
FIG. 2 is a schematic view showing a prior art arrayed waveguide grating type optical filter.
In FIG. 2, the reference numerals 01 and 02 indicate slab waveguide lenses, 03 is arrayed waveguides, and 06 is a bend waveguide block. As shown in FIG. 2, the slab waveguide lenses 01 and 02 are connected by the S-formed arrayed waveguides 03, and the entire circuit is point symmetrical. The S-formed arrayed waveguide is designed so that when the inflection point of the S form is directly connected, the individual waveguide lengths are equal. That is, the S-formed optical waveguide is constructed so that the optical length difference, which is required from geometrical layout, is once offset at the inflection point.
In the prior art embodiment shown in FIG. 2, the optical length difference required for filter operation is given by the bend waveguide block 06 in which the open angle is the same about the center of symmetry of the S-formed arrayed waveguides 03, spacings are constant, and the radius increases by a constant value.
With this construction, since the optical length difference of circuit is equal to the length difference between the adjacent waveguides of the bend waveguide block 06 (radius difference.times.open angle), a desired circuit can be designed which has a wide FSR, that is, a very short optical length difference. Therefore, in this configuration, the defect of the first prior art embodiment that the layout cannot be geometrically found is eliminated.
However, the configuration as shown in FIG. 2 has a problem that a width w of the circuit is large due to the point symmetry of the S-formed arrayed waveguide layout.
Therefore, there is a defect in that if the circuit size exceeds the effective substrate size, or even if can be laid out on the substrate, the number of circuits that can be disposed on a single substrate is decreased.
Since the number of circuits that can be produced from a substrate of the same size is an important factor for reducing the price and cost, the above problem reduces the application area of the optical filter.